An example of a prior art optical sensing device is given in U.S. Pat. No. 5,304,813. The device has a pair of linear arrays of light sources, each array arranged above the transfer path of a banknote, for emitting light towards the banknote, and a detector in the form of a linear array of photodetectors arranged above the transfer path for sensing light reflected by the banknote. The light source arrays have a number of groups of light sources, each group generating light of a different wavelength. The groups of light sources are energised in succession to illuminate a banknote with a sequence of different wavelengths of light. The response of the banknote to the light of the different parts of the spectrum is sensed by the detector array. Because each of the photodetectors in the array receives light from a different area on the banknote, the spectral response of the different sensed parts of the banknote can be determined and compared with stored reference data to validate the banknote.
In known banknote validators of the type such as described in U.S. Pat. No. 5,304,813, it is usual to pulse each light source for a predetermined time that is sufficient to let the detected signal rise to a stable value. The desirability to increase the speed of document processing calls for short pulses on the sources and short response times in the detector. Consequently, the bandwidth requirement in the detector circuit needs to be wide, typically of the order of 160 KHz, to avoid signal distortion. This in turn leads to a degradation of the signal to noise ratio, as noise increases in proportion to the square root of the bandwidth.
Typically, there are variations in the output for different light sources of the same wavelength for the same applied current, because of manufacturing variations. Also, sources of different wavelengths can have intrinsically different power outputs. For example, a green light source is capable of generating only approximately half the detector output of a red or infra-red light source. It is known to compensate for such variations by varying the current supplied to the source using a DAC and a current generator. Similarly, undesirable variations occurring in the detected signal can be accounted for by using a variable gain amplifier in the detector circuit.
A problem with adjusting the current supplied to the LEDs is that the output is not exactly proportional to the current for large changes, and also the LED wavelength changes with the intensity of the current.
It is desirable to provide a currency sensing apparatus which has a high signal to noise ratio and which can be produced at low cost.